The present invention relates to propeller propulsion devices, and more particularly to variable pitch propeller devices for marine craft such as inboard and outboard pleasure boats, yachts and fishing boats.
Variable pitch aircraft propellers are well known, implementations including hydraulic actuators being disclosed, for example, in U.S. Pat. Nos. 2,425,261 to Murphy et al., 2,554,611 to Biermann, and 4,362,467 to the present inventor. The '467 patent, which is incorporated herein by this reference, discloses a mounting flange for mounting to the propeller shaft flange of an engine, a hub for pivotally supporting a plurality of blades on respective radial axes, and a stationary annular hydraulic cylinder and piston between the mounting flange and the hub, the piston being connected by a yoke and transverse pin to a longitudinal rack member that engages respective pinions of the blades to rotate same through a wide angle of approximately 90.degree..
Typical marine propeller installations include a rearwardly extending propeller shaft on which is mounted a one-piece propeller having an annular hub portion, the shaft extending through the hub and threadingly engaging a retainer nut. The hub is secured against rotation relative to the shaft such as by splined engagement or by one or more keys or shear pins.
The aircraft propeller implementation of the '467 patent, while having certain advantages including the stationary annular hydraulic cylinder, is unsuitable for use in typical marine applications for a number of reasons. For example:
1. The shaft interferes with placement and movement of the yoke pin and the rack member;
2. The rear of the hub, including a biasing spring mechanism therein, interferes with access to the nut whereby the hub would be secured to the shaft;
3. The hub and blades would be difficult to remove for servicing and/or replacement in case of damage by underwater hazards; and
4. The device would be subject to water damage in that hub is unsealed, and the piston seals would have to operate in a wet environment.
A further problem exhibited in the prior art relates to the need in marine applications for means to decouple the propeller in case of impact with potentially damaging foreign objects such as submerged rocks and logs. Typically, such a device couples the propeller to it's shaft by a "shear pin" that transmits normal driving torques but which is supposed to sever when the propeller strikes an obstacle. The shear pins of the prior art are difficult to replace in that the propeller must be removed from the shaft, typically with significant difficulty resulting from interference with jagged edges of the severed shear pin. Moreover, the difficulty with which the propeller is removed significantly increases the risk of it's being dropped into the water.
Thus there is a need for a variable pitch marine propeller that is effective for providing a wide angular range, that is compatible with existing fixed-pitch installations, that is easy to service, repair, and replace, and is resistant to water damage.